Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
A dye is a colorant which is molecularly dispersed or solvated by a carrier. The carrier can be a liquid or a solid at room temperature. A commonly used carrier is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Water-based pigmented inks are prepared by incorporating the pigment in the continuous water phase by a milling and dispersing process. Pigmented inks require a water soluble dispersant in the pigment slurry during the milling process. Such a dispersant is necessary to produce a colloidally stable mixture and an ink that can be "jetted" reliably without clogging the print head nozzles.
Dispersing agents in an ink jet ink have the dual function of helping to break down pigments to sub-micron size during the milling process and of keeping the colloidal dispersion stable and free from flocculation for a long period of time.
A requirement in wide format ink jet printers is the delivery of at least 500 ml of ink through a printhead before nozzles begin to fail to fire ink droplets. Reproducible quantities of ink delivered prior to print cartridge failure (ink reliability) and particle size stability, over time, have been problems encountered with ink jet inks containing pigments as colorants and anionic dispersants.
U.S. patent application Ser. No. 09/351,614, filed Jul. 12, 1999, (Docket79666HEC) entitled "Color Pigmented Ink Jet Ink Set" discloses a typical ink jet pigmented ink. However, there is a problem with print cartridges containing those inks in that the print reliability over an extended period of time is not as good as one would desire.
U.S. Pat. No. 5,833,744 relates to an ink jet ink containing a surfactant which may be an aromatic sulfonate such as sodium dodecylbenzene sulfonate. However, there is a problem with using sodium dodecylbenzene sulfonate in that the print reliability over an extended period of time is not effective with all pigments, as will be shown hereinafter.
It is an object of this invention to provide an additive for a pigmented ink jet ink which would improve the print reliability. It is another object of this invention to provide an additive for a pigmented ink jet ink which would be useful with a variety of pigments.